1. Field of the Invention
This invention relates to power transmission belts and, more particularly, to a power transmission belt having a rubber body with an exposed outer surface and a fiber layer on at least part of the outer surface to reduce the coefficient of friction thereof.
2. Background Art
Toothed belts used for driving cam shafts and automobile injection pumps, such as timing belts, are often required to operate under severe conditions. This is particularly true in the automotive environment in which more powerful engines are being placed in more compact spaces in which the temperature may be very high. Yet, in spite of this, there is an increasing demand for belts to have even greater durability.
Toothed belts are also used for synchronized transmission in a number of industrial machines. It is generally required that the toothed belts consistently maintain accurate relative positioning of elements, often on relatively small machines over an extended time period.
The failure of toothed belts is generally classified into two types 1) rupture of the belt due to fatigue of load carrying cords; and 2) tooth chipping, often caused by overload and wear of a fiber cloth layer.
Improvements have been made which have reduced the incidence of failure of the load carrying cords. Among the improvements that have been made are: a) the use of aramid cords and high strength, relatively small diameter cords; b) use of hydrogenated nitrile rubber (H-NBR) compositions, which have excellent heat resistance properties; and c) the use of an auto tensioner for keeping belt tension constant both at startup and during operation.
Attempts have also been made to address the problem of tooth chipping. One attempted solution involves the use of cloth layers on the teeth, which cloth is made of high tension type nylon 6-6 and aramid fibers. However, this attempted solution has not been completely effective. To further improve the chip resistance of a toothed belt covered with a cloth layer, efforts have been made to decrease the frictional coefficient on the surface of the cloth.
In EPO 662571B1,there is disclosed a toothed belt in which a polymer matrix layer containing a fluorine resin is coated on the outside of a fabric layer covering the teeth, as by spraying or coating. The fluorine resin is bonded with no boundary layer in a special polymer matrix. In turn, the polymer matrix is bonded to the cloth layer. However, since the fluorine resin is firmly bonded to the polymer matrix, the fluorine resin remains surrounded by the matrix and may not adequately provide the desired effect of reducing the frictional coefficient. Further, the fluorine resin can be incorporated in only small amounts, as a result of which the polymer matrix layer is relatively thin. As a result, adequate tooth chipping resistance may not be obtainable using the fluorine resin.
In Japanese Patent Laid-Open No. 7-151190, a rubber mixture is disclosed containing fibrillated fluorine resin that is applied to a fabric layer so that the fibrillated fluorine resin is present at oppositely facing surfaces of the fabric layer and impregnates the fabric: between the oppositely facing surfaces. The surface of the cloth, facing oppositely to that surface which is exposed on the completed belt, is bonded through an adhesive layer to rubber on the belt body. The fluorine resin may be fibrillated by a kneading step so that the fluorine resin in the rubber is not in a form that diminishes the strength of the rubber mixture. However, the fluorine resin fibrillated in the rubber mixture may not be present in a sufficient quantity at the friction generating surface to reduce the coefficient of friction to the degree desired.
Further, in order to fibrillate the fluorine resin and allow it to be present in a form that does not weaken the rubber mixture, the fluorine resin can be included in amounts of only about 1 to 30 parts by weight per 100 parts by weight of rubber impregnated into the cloth. As a result, tooth chipping resistance may not be sufficient.
V-belts and V-ribbed belts used in power transmission in general industrial machines and in the automotive market commonly have a fabric attached to the back surface of the belt to both reduce friction between the belt back surface and a cooperating pulley and avoid generation of unwanted noise as the pulley contacts the back surface in operation. Ideally, wear resistance is improved, while noise generation is minimized by reducing the frictional coefficient between the fabric and a cooperating pulley. However, to date, additional improvement is sought in both of these areas.